1. Field of the Invention
This invention relates to a Doppler signal frequency converter. More particularly, this invention relates to an improved Doppler signal frequency converter for use in an apparatus employing ultrasonic waves to detect and measure the velocity of movement of an object.
2. Description of the Prior Art
Doppler pulse devices are widely used wherein an ultrasonic pulse beam is transmitted into an object at a fixed repetition frequency and the reflection from the object is picked up and the distance to the object is measured by comparing the time difference between the transmitted signals and the received signals, and at the same time the velocity of movement of the object is detected and measured by detecting changes in the frequency of the received signal.
Generally, the repetition frequency of the pulse beam is selected according to the distance to the object. However, in the case of measurement of an object which is distantly located, if the frequency selected is high compared with the repetition frequency selected on the basis of the distance to the object of examination, it is well-known that an aliasing echo is produced which indicates that the object is closer than is actually the case, making discrimination of the distance difficult.
A similar phenomenon is seen in the case of measurement of the velocity of the object. If the repetition frequency selected is low compared with the Doppler frequency arising from the velocity of the object, the aliasing echo results in a low frequency, making discrimination of the velocity difficult.
In order to measure both distance and velocity without production of aliasing echo, it is known that the relationship between a maximum Doppler frequency fd and pulse repetition frequency fr has to conform to fd=fr/2 in the case of a device able to detect not only absolute velocity but also whether it is positive or negative, and fd=fr in the case of a device which detects and measures only the absolute velocity.
Here, in the case of a device able to discriminate whether velocity is positive or negative, from EQU fd=fo.multidot.k.multidot.V=fr/2
(fo: ultrasonic pulse beam frequency, PA0 k: constant, V: maximum velocity)
the maximum measurable velocity V is EQU V=fr/(2fo.multidot.k) .multidot.
However, as can be understood from the above formula, there is a drawback that if the pulse repetition frequency fr is increased in order to increase the maximum measurable velocity V, there is a decrease in the maximum distance at which the object can be measured without the production of aliasing echo, therefore measurement of the velocity is not possible in the case of a distant rapidly-moving object.
Another problem has been that if a low ultrasonic beam frequency is selected, not only is it difficult to produce a transmission wave with a narrow pulse width, but it is also impossible to form a finely focussed beam, the result being a decrease in distance resolution and directional resolution, thus rendering impossible simultaneously to establish the distance to and the velocity of a distantly located rapidly-moving object.